The customary contact lenses are hard at least in one stage of the manufacturing process. The rim and surface can therefore be machined for instance by turning or grinding.
Usually, the hard contact lenses made of polymethylmethacrylate or other plastics are usually machined and subjected to chip removal. After making the face and reverse side the rim is given the desired contours by turning or with a sharp blade followed by polishing. The lens then receives the final surfaces in a punch molding operation. The edge is however still thick and sharp edged and therefore must likewise be machined upon removal of the chips as decribed above.
In case of strong minus lenses a heavy edge occurs in the lens. These lenses must therefore receive a so-called lenticular shape, that is, the optically effective area of the lens must be kept as small as possible and as much matertial as feasible must be removed from the outer surface starting from the edge in order to obtain a physiologically acceptable edge thickness.
This type of processing is, as explained, possible only with hard materials. If the material is not hard at some stage of the manufacturing process, these metals can no longer be used. Such material is for instance silicone elastomer which, for various reasons, is a preferred material for making contact lenses.
The silicone elastomer material is soft already in its initial stage. It is then subjected to a vulcanization in order to obtain a three-dimensional cross-linking. After the vulcanization which is effected by means of pressure and heat, the finished product, in this case the cool contact lens is removed from the mold with its front and reverse side already in final shape. The finished product however is still more or less soft in view of the characteristics of the silicone elastomer and cannot be further processed.
In order to make a contact lens from this material where the lens is intended to have a higher degree of negative power of refraction a die punch is therefore necessary which already embodies the outer contours for making the lenticular shape. Such die punch must have a lesser degree of curvature in its central zone than in the marginal zones.
To make this kind of die punch by the established methods of precision mechanics and optics is however impossible. By these methods normally only spherical surfaces can be formed. The tools used in the course of these methods because of the divergent curvature required in the punch will impinge either against the flat or the steeper zone. For this reason the surfaces cannot be formed by coarse or fine grinding.
According to an earlier invention of the present inventor the die punch is made by first forming a convex negative and then copying the concave die punch from such negative by means of a copy method. For the copying, various methods are available such as for instance galvano-plastic reproduction. All of these methods however require a subsequent machining of the die positive in order to obtain an optically perfect surface. These processes are therefore of value for the industrial production only if many contact lenses are obtained by punching from one positive. For making a single contact lens or a few infrequently required lenses, the earlier invention would be too costly.
It is therefore an object of the present invention to provide a process for obtaining die positives by simple reproduction in cases where the die positives cannot be produced by a direct method and to provide for such method of reproduction from easily formed negatives.